DE69215641T2 - Pressure swing adsorption process for low temperatures with cooling - Google Patents

Description

Background of the invention Field of the invention

The invention relates to the field of gas separation. In particular, it relates to the improved production of oxygen from air.

Description of the state of the art

Adsorption processes have been widely used for the separation and purification of gases. High surface area sorbents have an affinity for adsorbing gas molecules on their surface. The amount of gas adsorbed depends on the specific sorbent used, the gas adsorbed, and the temperature and pressure conditions under which the adsorption process is carried out. For most sorbents, the amount adsorbed increases with increasing partial pressure of the adsorbed gas component and with decreasing adsorption temperature. Consequently, the amount of gas adsorbed can be increased by lowering the adsorption temperature. In most practical applications, it is necessary to desorb the adsorbed gases to regenerate the sorbent to allow the adsorption process to be repeated on a cyclic basis. The desorption step works best at high temperatures and low pressures. For such practical applications, therefore, either the pressure or the temperature or both must be changed or "changed" on a cyclic basis between the adsorption and desorption steps. These two basic approaches to gas separation are called pressure swing adsorption (PSA) and temperature swing adsorption (TSA).

In recent years, PSA processes have been developed for the production of oxygen and nitrogen from air. In such processes, feed air is fed into an adsorption bed containing a sorbent that can selectively adsorb the more easily adsorbable component from air, i.e. either nitrogen or oxygen, while the more difficultly adsorbable component is discharged from the adsorption bed. While the behavior of such PSA processes clearly depends on the temperature conditions at which the adsorption and the desorption, most PSA processes have been designed to operate generally at ambient temperature conditions without using any special arrangement to control the temperature conditions during the adsorption process.

DE-A-2 221 379 discloses a method and a system for separating a gas mixture according to the preamble of claim 1 and claim 11, respectively. However, in this known adsorption system an adsorption vessel is used which has two adsorption beds between which a thermal buffer section is arranged.

In PSA systems, heat is released during adsorption and heat is absorbed by the sorbent during desorption. The temperature of the adsorption bed therefore tends to rise during the adsorption step while the temperature of the bed falls during the desorption step. The temperature change is most pronounced during the portion of the overall PSA cycle in which the adsorption bed is pressurized to an upper adsorption pressure or depressurized to a lower desorption pressure, provided that the adsorbent is substantially free of strongly adsorbed contaminants that can only be effectively desorbed by purging and that prevent adsorption of less strongly adsorbed components. The pressurization and depressurization of the open gas spaces in an adsorption system, such as the manifold assembly or the head spaces in vessels containing the bed of sorbent material, also cause temperature changes due to the reversible work done by the compression and expansion of the gases therein. In a dynamic process such as a PSA process, a large portion of the heat of adsorption and compression is transferred to the flowing stream of feed gas, e.g. air, and is carried out of the adsorption bed. In a typical PSA operation, such as that used for the production of oxygen and/or nitrogen from air, the forward flow of gas during adsorption exceeds the reverse flow of gas during desorption. Consequently, there is a net forward flow of enthalpy which tends to reduce the average temperature of the adsorption beds used in a PSA system if the temperature oscillations therein are greater than in the region near the entrance to the beds.

The effect of temperature on PSA processes for producing oxygen from air is discussed by Izami et al. in "High Efficiency Oxygen Separation with Low Temperature and Low Pressure PSA", AIChE, San Francisco, California, Nov. 1989. Five different molecular sieve type sorbents that could selectively adsorb nitrogen from feed air were investigated in this study, including Na-X (with two different Si/Al ratios), Ca-A, Ca-X and Si-X. It was found that the sorbents containing alkaline earth cations (calcium and strontium) showed the best N2/O2 separation factors at approximately room temperature, whereas the separation factor for the Na-X sorbents was maximum at about -30ºC. In all cases, the nitrogen holding capacity increased with decreasing temperature, as expected from adsorption theory discussed above. Proof process tests with Ca-A and Na-X sorbents confirmed that the Ca-A sorbent was most efficient between 0ºC and room temperature, whereas the Na-X sorbent was most efficient at temperatures well below 0ºC. In these tests, the adsorption beds were thermostatted and effectively held at a fixed temperature. Larger scale pilot tests were also conducted with Na-X adsorbent. Cooling coils were provided in the bed and a heat regeneration section was also used between a drying section for drying the incoming feed air and the adsorption bed to achieve bed temperatures lower than the feed gas stream temperatures. Such tests confirmed that adsorption efficiency increased and costs decreased when the adsorption temperature was reduced to a nominal value of -15ºC. The tests were conducted under near adiabatic rather than isothermal conditions and temperatures were not uniform. These tests demonstrated that it is advantageous to operate the PSA process with Na-X adsorbent at sub-ambient operating temperatures. External cooling was used to achieve the desired low adsorption bed temperatures. An optimum desorption pressure of about 0.3 bar (0.3 atmospheres) was also used.

In contrast, however, other authors have found that low adsorption bed temperatures adversely affect PSA system performance. Collins disclosed in US-A-3,973,931 that very large axial temperature variations can occur in superatmospheric PSA processes for producing oxygen from air. Temperature variations of more than 50 ºC were observed in adsorption beds containing zeolitic molecular sieve material. It was found that a very large temperature gradient occurred near the feed end of the bed, resulting in a temperature minimum at a distance of about 0.3 m (1 foot) from the feed end of the bed, with temperatures gradually increasing over the remainder of the bed. After repeated adsorption-desorption cycling operation, the temperature profile remained consistent with little variation with each cycle. Collins found that these temperature variation conditions were detrimental to the purity and yield of oxygen using such superatmospheric PSA cycles. Consequently, US-A-3 973 931 teaches that improved operation results from heating the feed stream by at least 20 ºF (11 ºC). Although the operating data presented show that there is a large axial temperature variation, the minimum bed temperature is thereby raised, as are the temperatures throughout the rest of the adsorption bed. In US-A-3,973,931, the temperature drop at the inlet end is attributed to an "unintended heat recovery step" and it is shown that the temperature drop is greatest when water vapor contaminants from the feed stream are adsorbed in this inlet region of the bed. In US-A-3,973,931, various means of raising the feed stream temperature are suggested, including controlling or partially bypassing the aftercooler of the feed air compressor. The heat of feed air compression is more than sufficient to produce the somewhat higher feed air temperatures used for the improved process taught in US-A-3,973,931.

Thus, in the field of PSA air separation, there are differing teachings regarding the choice of adsorbent, pressure levels for adsorption and desorption, and recommended operating temperatures, with temperatures both above and below ambient temperatures being recommended. Nevertheless, most commercial PSA air separation processes, as described above, are operated under ambient conditions without temperature control and without special attention to thermal effects that occur during the cyclic adsorption-desorption processes.

There is, of course, a desire in the art to improve PSA operations to more fully satisfy the ever-increasing demands of practical commercial separation operations for air and other gases. Such a desire in the art relates in particular to increasing the yield of oxygen or other desired products with advantageous PSA systems that utilize rather than disregard thermal effects that occur during cyclic PSA operations. For such improved operations, however, it is desirable that the PSA systems avoid the use of relatively expensive auxiliary equipment, such as the external cooling used in accordance with the teaching of Izami et al.

It is an object of the invention to provide an improved PSA process and an improved PSA apparatus for the production of oxygen from air and other desirable gas separations.

It is a further object of the invention to provide a PSA gas separation apparatus and process which exploit the thermal effects occurring during the cyclic adsorption-desorption PSA sequence to avoid the need for external cooling.

It is a further object of the invention to provide a PSA method and system for improving the overall efficiency and economy of oxygen production from feed air.

With these and other objects in mind, the invention is described in detail below, with the novel features being particularly pointed out in the appended claims.

Summary of the invention

The invention relates to a PSA process and system in which means are provided for the controlled retention of internally generated self-cooling so that the average temperature of the adsorption bed is reduced. The overall efficiency and economy of the air separation process is thereby increased.

Short description of the drawings

The invention is described below with reference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of an embodiment of a PSA adsorption bed with self-cooling according to the invention; and

FIG. 2 is a process flow diagram of a typical two-bed PSA system.

Detailed description of the invention

The objects of the invention are achieved by a PSA process and system at a sub-ambient bed temperature, wherein the required cooling is provided internally without the need for externally supplied cooling. By thus retaining and utilizing internal cooling effects of the PSA cycle so that the average temperature of the adsorption bed is reduced, the overall efficiency and economy of the PSA process for separating air or other gases is increased.

The desired high performance oxygen recovery from air and other high performance gas separations are achieved in the practice of the invention using a PSA system as described herein, comprising a zeolite molecular sieve adsorbent, e.g. of the Na-X type, and operated at adsorption-desorption pressure conditions as described herein. The forward flow of enthalpy exceeds its reverse flow, thereby producing a net cooling which effectively reduces the average temperature of the adsorption beds. The loss of such cooling power is prevented by the combination of vessel insulation, backfilling of the end spaces in the adsorption vessels containing the adsorption beds, and the use of heat recovery regions at the feed end of the beds, typically the lower end, and between the desiccant commonly used to dry the incoming feed air and the adsorption beds. In this way, the invention does not require the use of external cooling. The amount of self-regenerated cooling retained can be controlled via the design characteristics of the heat recovery regions and the degree of insulation used. Fine tuning and control of the bed average temperature is achieved by controlling the temperature of the feed gas, e.g. air, which is achieved by adjusting the degree of cooling achieved in the compressor aftercooler.

In the equilibrium PSA process achieved using the zeolite molecular sieve adsorbents described herein, the more readily adsorbable or heavier component(s) of the feed gas passed to the bed at an upper adsorption pressure are selectively adsorbed and form an adsorption front which advances from the feed end of the bed to the product end thereof, while the less readily adsorbable or lighter component(s) are passed through the bed and recovered from the product end thereof at an upper adsorption pressure for further processing and/or downstream use. In such an equilibrium process, nitrogen is the more readily adsorbable component and oxygen is the less readily adsorbable component of the feed air. Upon completion of this adsorption step, the bed may also optionally be cocurrently depressurized to an intermediate pressure by releasing gases from the product end of the bed, with the released gas being used to equalize pressure with another bed in the system and/or as a purge gas for another bed. During this time, the adsorption front of the readily adsorbable component continues to advance towards the product end of the bed, but without breakthrough. The bed is then countercurrently depressurized to a lower desorption pressure by releasing gas from the feed end of the bed, with or without subsequent purging at the lower desorption pressure value to enhance desorption and removal of the more readily adsorbable component, e.g. nitrogen in air separation, from the bed. After completion of this desorption/purging step, the bed can be partially repressurized to an intermediate pressure by introducing an oxygen-rich product at the product end of the bed. Subsequently, the bed is pressurized to its upper adsorption pressure when the cyclic PSA process sequence is continued with additional amounts of feed air being passed to the bed during each subsequent adsorption step. In a typical PSA cycle of the type described above, the forward flow of gas exceeds the reverse gas flow in the bed, resulting in a net forward flow.

The temperature of each adsorption bed varies with position and time during PSA cycling. It has been found that pressure changes have a dominant effect on the local temperature within the bed. Lowering the pressure typically lowers the temperature of both the gas in the bed and the adsorbent. Lowering the pressure in the open gas spaces in the adsorption vessel also lowers the local gas temperature therein. The temperature reduction at reduced pressure causes the average temperature of the returning low pressure gas stream to be lower than that of the forward high pressure gas stream. For typical PSA cycles of the type described here, it has been found that the forward flow of enthalpy initially exceeds the reverse flow of enthalpy, and that a net forward flow of enthalpy orders out of the bed through its product end. While such conditions persist, the bed temperature tends to decrease until the enthalpy flows balance and a dynamic steady state is established. While most systems were found to tend toward a steady state, some examples were observed where uncontrolled temperature drift occurred when appropriate means of temperature control were not used.

The local temperature shift in the bed is greatest where the local change in total adsorption of the more readily adsorbable component, and a portion of the less readily adsorbable component, onto the adsorbent is greatest. In the PSA oxygen process of the present invention, the greatest adsorption change due to the adsorption of nitrogen, and some oxygen, occurs near the feed end of the bed after removal of water vapor and carbon dioxide from the feed air in the drying section of the bed. Upstream of this region of greatest adsorption change in the bed, the adsorption of water vapor and carbon dioxide generates heat and can also cause large temperature changes, but these changes occur primarily in such a way that the forward gas stream and the reverse gas stream have nearly equal average temperatures. However, to return the cooling generated as a result of desorption in the drying section of the bed, a thermal regenerator zone is located at the feed end of the bed upstream of the drying section of the bed, as shown in FIG. 1. In the nitrogen adsorption section, the reverse flow can account for up to 90% of the forward flow from the feed end to product of the bed Such conditions, combined with large temperature differences between the forward and backward flowing gas streams, lead to a large drop in the local bed temperature. These differences in the thermal behavior of different zones lead to large axial temperature gradients in the beds, which are increased by the regenerators according to the invention.

In FIG. 1, a self-cooling PSA bed according to the invention is shown arranged within an adsorption vessel, generally designated by the reference numeral 1. This vessel has a feed air inlet line 2 through which feed air flows to a lower manifold assembly 3. A first thermal regenerator zone 4 referred to above is arranged upstream of the manifold assembly 3, i.e., below a drying section 5. The downstream, i.e., upper, region of the drying section 5 has a second thermal regeneration zone 6, as described herein. An adsorption bed 7 is located above the drying section 5 within the vessel 1 and constitutes the main region therein for the desired selective adsorption of nitrogen from air. Above the adsorption bed 7, the vessel 1 has an upper manifold assembly 8 and an oxygen product outlet line 9. Insulated walls 1A are provided for the vessel 1 to prevent cooling loss from the vessel in conjunction with other elements of the invention. Such other elements include filling the lower manifold provided here and the use of one or two thermal regenerator zones in particular embodiments of the invention. One such regenerator zone is located at the feed end of the vessel immediately downstream of the lower manifold and the other regenerator zone comprises the downstream region of the predrying section, if provided, immediately upstream of the main adsorption bed region used for the desired air separation purposes.

The temperature shift of a bed of zeolite molecular sieve material, i.e. Na-X type material, initially in equilibrium with air is generally about -5ºC when the bed pressure is changed from 125 kPa to 50 kPa. For pure nitrogen subjected to the same pressure reduction, the temperature shift would be about -6ºC, while for pure oxygen the shift would be only about -2ºC. This is to be expected since nitrogen is more strongly adsorbed and has a greater heat of adsorption than oxygen. For the designated shift of -5ºC, with a reverse flow of about 90% of the forward flow, the temperature drop was determined to be about -45ºC. This drop probably occurs near the feed end of the nitrogen adsorption zone, i.e. adsorption bed 7 of FIG. 1, with smaller temperature drops occurring further downstream in the bed. In this way, a substantial amount of internal cooling is spontaneously generated during such transatmospheric PSA process cycles. It turns out that such internally generated cooling is even larger for superatmospheric high pressure PSA cycles, confirming Collins' observations.

In the illustrative practice of the invention, to control the retention of internally generated self-cooling and reduce the average temperature of the adsorption bed, simulations based on adsorption-desorption pressures of 125 and 50 kPa, respectively, were used for PSA processing operations using zeolite Na-X type molecular sieve adsorbents to achieve oxygen production of 15 tons per day at 93% oxygen purity.

For example, the insulated PSA vessel includes a lower plenum having a height of about 6 inches (15.2 cm), a 1 inch (2.54 cm) regenerator section, an 8 inch (20.3 cm) desiccant section containing desiccants for removing water, carbon dioxide, heavy hydrocarbons and the like, a 50 inch (127 cm) adsorbent section for selectively adsorbing nitrogen from feed air, and an upper plenum of about 7 inches (17.8 cm). Conventional insulation having a thickness of about 2 inches (5.1 cm) was used to minimize cooling loss in the vessel. For purposes of the invention, the lower plenum or manifold assembly is filled with 5/16 inch (0.8 cm) conductive brass balls to prevent thermal cycling due to the work of compression and expansion of the gas therein. The balls serve to reduce the total gas volume therein by on the order of about 60%, thus directly reducing the compression and expansion work, and the thermal cycle by the same amount. Furthermore, the conductive balls absorb heat from the gas and release heat from it, thus reducing the thermal fluctuations even further. It was found that the rise in nitrogen adsorbent temperature due to the compression and expansion work in the manifold assembly is proportional to the product of the manifold assembly void volume and the pressure swing divided by the net oxygen product rate. Thus, the manifold assembly void volume should be kept low compared to the net product flow rate, especially when the difference between adsorption pressure and desorption pressure is large.

The first or lower regenerator section is filled with copper balls with a particle size of 1.68 to 2 mm (10x12 mesh) held between separating grids to prevent loss of the balls or ingress of adsorbent particles from the adsorbent bed section of the vessel. This regenerator section can Collect low level cold from the downward flowing gas streams and transfer it to the upward flowing gas streams above. This is required when the drying section must deal with high concentrations of water and other impurities in the feed air which have large heats of adsorption and when a drying section is not used. This regenerator section is also required if the desiccant used in the drying section is capable of adsorbing significant amounts of nitrogen or oxygen/argon from the feed air stream.

As indicated above, the desiccant layer is required to remove strongly adsorbed contaminants, such as water vapor, carbon dioxide, and heavy hydrocarbons, before they reach the nitrogen adsorbent region of the adsorbent vessel. Such contaminants are more difficult than nitrogen to desorb from the nitrogen selective adsorbent and would thus reduce the performance of the PSA system. Furthermore, such contaminants would reduce the thermal cycle in the nitrogen adsorbent region and would thus reduce the self-cooling effects desired in the practice of the invention. On the other hand, the desiccant should not adsorb significant amounts of nitrogen, oxygen, or argon, since such undesirable adsorption would reduce the performance of the desiccant layer. In typical practice, the desiccant layer is operated primarily near the temperature of the feed air stream, i.e., at a temperature higher than that of the nitrogen adsorbent region.

If the desiccant used is alumina or another relatively heavy adsorbent, the desiccant particles used can be smaller than those used for the nitrogen selective adsorbent because they are not as easily lifted and entrained by the upward flowing streams.

No separate second regenerator section is required between the desiccant layer and the sorbent bed if the first regenerator section is working well, since the upper region of the desiccant layer itself acts as a sufficient regenerator section, while also contributing to completing the removal of the strongly adsorbed contaminants.

The nitrogen selective adsorbent section is filled with beds of Na-X zeolite with a particle size of 1.68 mm to 2.38 mm (8x12 mesh) with a silica/alumina ratio of about 2.0 with less than 5 g/kg water load.

The upper manifold assembly is packed with balls or other objects to reduce compression energy loss, but it is not necessary to increase the thermal Such thermal fluctuations do not significantly affect the self-cooling of the nitrogen adsorbent region of the vessel. It should also be noted that no regenerator is required between the upper manifold and the nitrogen adsorbent region since little heat is conducted from the upper manifold downward into the nitrogen adsorbent region in any event.

Since the main temperature drop occurs between the desiccant layer and the nitrogen adsorbent region, which must operate at a low temperature, it may not be readily apparent why, in practice, a first regenerator section is located in the vessel below the desiccant layer. The reason for locating this first regenerator section below the desiccant layer is to prevent heat of adsorption from water from pumping heat into and through the desiccant layer and then into the nitrogen adsorbent region. Water adsorbed in high concentration from the feed air entering the desiccant layer causes a temperature rise at the lower end of the desiccant layer. The feed air stream absorbs the heat as enthalpy and carries it upward a short distance. During the low pressure, downward flow steps of the overall PSA cycle, desorption of water cools the lower end of the desiccant layer and cools the downward flowing gas. The gas carries the cold down a short distance. The heating and cooling effects are equal when the process reaches steady state. However, since most of the cooling occurs at the entrance to the desiccant layer, some of the cooling leaves the desiccant layer during each PSA processing cycle. This cooling would be lost with the exhaust gas leaving the adsorption vessel unless the first regeneration section is arranged to adsorb the cooling from the exhaust gas and store it for advantageous use in practice. The first regenerator section is designed to store the cold and then return it to the desiccant layer with upwardly flowing gas during the next step of the PSA cycle. When this is done, the water adsorption heat is balanced by the recovered cold and this adsorption heat does not pass up through the desiccant layer to the nitrogen adsorption region of the vessel.

It is understood that the upper end of the desiccant layer is confronted with only low concentrations of water or other strongly adsorbed contaminants, and thus acts simultaneously as an efficient second regenerator section. As a regenerator, the upper end will readily absorb the cold generated in the nitrogen region and to the feed gas stream flowing upward in that region of the vessel during the next portion of the cyclic processing operation.

In the teaching of Izami et al. referred to above, a regenerator region is placed only between the desiccant layer and the nitrogen adsorption region. This is much less effective than the arrangement of the invention because at least some of the cold generated in the desiccant layer is lost and this loss leads to higher temperatures in the nitrogen adsorption region regardless of the regenerator efficiency. As mentioned above, the top of the desiccant layer simultaneously functions to remove traces of strongly adsorbed contaminants and to serve as a regenerator. These two functions of the desiccant layer are not mutually exclusive but instead provide a highly desirable synergistic effect for more efficient operation of a PSA process and system.

In order to substantially reduce the void space when filling the lower manifold assembly of the adsorbent vessel, it is important that the particles used, such as the conductive particles described above, be large enough to avoid creating large pressure drops which would significantly contribute to the overall adsorbent pressure drop or cause flow maldistribution through significant lateral pressure gradients in the end space.

It will be understood that conductive elements other than the 1.68 to 2.0 mm (10x12 mesh) particle size copper balls mentioned above may be used in the first or lower regenerator sections. Preferably, the regenerator is filled with conductive elements comprising metal particles having a thermal conductivity somewhat lower than copper. For example, materials having a conductivity of about one-half to about 1/10 that of copper are desirable so that axial conductivity is reduced without undue reduction in gas-to-solid thermal exchange efficiency. Any substantial further reduction in the thermal conductivity of the material in the first regenerator section would serve to reduce regenerator performance and would require some increase in the depth of the first regenerator section. For purposes of the invention, the conductivity of the material in the first regenerator should be selected so that, together with the amount and size of such material used, it allows storage of cold in the return stream during the desorption portion of the processing sequence carried out in the adsorbent bed. Such conductivity is preferably from about 430 to about 0.85 W/m K (about 250 to about 0.5 BTU/ºF/ft/ft²/hr), preferably from about 260 to about 26 W/m K (about 150 to about 15 BTU/ºF/ft/ft²/hr). Aluminum, Steel and Cast Iron are suitable materials for this purpose, as well as copper as described above. Loosely packed regenerator particles should be sized to prevent entrainment during upward flow of gas or horizontal movement during horizontal gas flow. The particles may be selected to be at least the same size and density as the particles used in the adsorbent region. Regenerators in the form of shields, grids or similar structures are not subject to such entrainment and can be subjected to greater forces without inadvertent movement occurring.

It is within the scope of the invention to use regenerator sections other than the cylindrical axial flow type mentioned above. For example, grid stacks may be used and, if desired, they may be separated by thin layers of spheres or other particles of low or moderate thermal conductivity. The regenerator material need not be in spherical form and may be composed of pellets, irregular particles, fiber mats, porous plates or particles formed into porous structures by sintering or bonding. In a radial flow adsorption system, the regenerator may be in the general form of a cylindrical layer which divides the feed end manifold assembly, either externally or internally.

A first regenerator made of conventional plate and fin cores is not desirable in the practice of the invention because of the resulting increased axial conduction. The first regenerator should in any event have a moderately low void volume in order to avoid thermal cycling due to the existence of reversible compression and expansion work in the regenerator itself. As mentioned above, locating the first regenerator below the desiccant layer is an important feature of the invention and this is of particular importance when the water content of the feed air is high.

It is to be understood that in the invention the use of NaX zeolite is not limited to the embodiment having a silica/alumina ratio of about 2.0 as mentioned in the above illustrative example. It is within the scope of the invention to use NaX zeolites having silica/alumina ratios of about 2.0 to about 2.6, preferably less than about 2.4, e.g. about 2.0 to about 2.4. The water load of the NaX zeolite used as an adsorbent in the practice of the invention should be less than about 25 g water per kg, preferably less than about 10 g water per kg, most preferably less than 3 g per kg. It is also within the scope of the invention to use zeolites of type 5A or 4A. Such adsorbents are only moderately highly nitrogen-selective equilibrium type adsorbents. Strongly nitrogen-selective adsorbents of the equilibrium type, which are prepared by ion exchange of NaX- Zeolites, such as LiX and CaX, should not be used in the low temperature region of the adsorbent beds because desorption of nitrogen from such adsorbents becomes difficult at the low temperatures achieved in self-cooling cycles. However, such highly nitrogen-selective adsorbents can be used to advantage in the higher temperature regions near the product end of the bed. Rate-selective adsorbents, such as carbon adsorbents, which selectively adsorb oxygen instead of nitrogen on a rate-selective basis, should also not be used because it is more difficult to produce the required self-cooling in efficient air separation cycles using such adsorbents.

It should be noted that feed air to be separated in the practice of the invention usually contains water vapor, carbon dioxide and other strongly adsorbed contaminants which are removed by the desiccant layer. On the other hand, if the feed air is free of such contaminants, the desiccant layer may be omitted. In such a case, all other elements of the PSA system of the invention would continue to be used as described above and shown in FIG. 1, including the first regenerator section located above the lower manifold assembly. These elements would be sized as described above for the illustrative example. In this case, the first regenerator section would serve to perform the regeneration function of the upper section of the desiccant layer and would thus block or prevent the loss of desirable refrigeration from the nitrogen adsorbent region of the vessel into the lower manifold assembly.

The invention may be practiced in PSA systems having one or more adsorbent beds, with systems having two to four beds generally being preferred, although systems having a greater number of beds, up to 10 to 12 beds or more, may also be used. FIG. 2 illustrates a typical two-bed PSA system used for the desired air separation to produce oxygen product. In this embodiment, feed air in line 10 is compressed in an air compressor 11 and passed to an aftercooler 12 for cooling before being passed to either an adsorbent bed 13 or an adsorbent bed 14, depending on the portion of the overall PSA processing sequence to be carried out in the beds at any given time in the overall cycle. A line 15 containing a valve assembly 16 is adapted to pass feed air to line 17 for transfer to the feed end or lower end of bed 13. The line 17 is also connected to a line 18 containing a valve arrangement 19 for the purpose of removing excess Nitrogen from bed 18 is connected to a line 20 for release from the system. Similarly, a line 21 containing a valve assembly 22 is provided for passing feed air to a line 23 for introduction into the lower end of bed 14. A line 24 containing a valve assembly 25 is provided for passing excess nitrogen gas to line 20 for release from the system. It will be understood that this excess nitrogen stream contains the more readily adsorbable nitrogen component which is desorbed and removed from the bed during the lower pressure desorption step of the process.

At the lower end of the bed 13, a line 26 containing a valve arrangement 27 is provided for conducting the more difficultly adsorbable component of the feed air, i.e. oxygen, which is removed from the upper portion of the bed 13, to a line 28 for recovery as the desired oxygen product of the air separation process. Similarly, a line 29 containing a valve arrangement 30 is provided for similarly conducting more difficultly adsorbable oxygen from the upper portion of the bed 14 to the line 28 for recovery as the oxygen product gas. It will be understood that a line 31 having a valve arrangement 32 is provided to provide fluid communication between lines 26 and 29 so that gas conducted from the upper portion of a bed undergoing relaxation from its upper adsorption pressure can be conducted to the other bed initially at a lower pressure for pressure equalization between the beds so that the pressure requirements of the cyclic sequence of higher pressure adsorption - lower pressure desorption in each bed can be minimized.

In very large PSA air separation plants, several adsorbent beds can be connected for parallel flow into a processing bank. All of these beds then undergo the same processing sequence together and simultaneously. Each individual bed in a given bank desirably shares common feed and exhaust piping, having suitable control devices to control flow between the beds. Such an adsorption bank can contain any number of adsorbent beds, however, each bank in a given PSA adsorption system would contain the same number of beds as the other associated banks. Any suitable number of banks can be used in a PSA system, with two and four banks generally being preferred.

It will be understood that the invention may be practiced using various modifications of the underlying adsorption-desorption-repressurization processing sequence depending on the overall requirements of any particular air separation operation. A particular processing sequence is described below. It will be understood It is contemplated that each bed of the PSA system subjected to this particular processing sequence or any other such sequence will have the configuration described above in connection with FIG. 1, except that the desiccant layer may be omitted as described above. Thus, all beds are designed for self-cooling low temperature operation, with no external refrigeration source being used to achieve the desired low temperature operation.

Processing cycle sequence

Step 1 - Pressurize the bed to the upper adsorption pressure by introducing feed air into the feed end of the bed;

Step 2 - adsorption at the upper adsorption pressure, whereby feed air is introduced into the feed end of the bed and less selectively adsorbable oxygen is withdrawn from the product end of the bed as the desired product gas;

Step 3 - cocurrent flashing with release of void gas from the product end of the bed to reduce the pressure of the bed to an intermediate value, the released gas being introduced into another bed of the system to serve as a purge gas or for pressure equalization with an initially lower pressure bed, or the released gas being recovered as an oxygen-rich secondary product;

Step 4- Countercurrent expansion with release of gas from the feed end of the bed, which is expanded to the lower desorption pressure, the released gas being oxygen-poor exhaust gas;

Step 5 - purging at the lower desorption pressure with oxygen-rich reflux gas from another bed which is introduced into the product end of the bed and removing additional amounts of oxygen-poor off-gas from the feed end of the bed; and

Step 6 - partially repressurizing the bed by introducing gas released from the product end of the other bed into the product end of the bed being repressurized, increasing the pressure of the bed being repressurized from the lower desorption pressure to an intermediate pressure before being further pressurized to the upper adsorption pressure as the processing sequence continues on a cyclical basis in each bed in the PSA system.

It is understood that the oxygen-rich reflux gas withdrawn from the product end of the bed in step 3 and used for purging and/or pressure equalization purposes may be passed directly to another bed in the system for such purposes and/or stored in a separate storage vessel for such use. In one embodiment, gas released from a bed during cocurrent expansion thereof may be used initially for pressure equalization purposes, either partially or fully, with additional quantities of gas so released being used to pressurize a storage vessel used to provide purge gas to another bed in the system at a later time, with further quantities of gas being used directly to purge another bed in the system.

In the practice of the invention, in which PSA containers are used as in the container example described above, with gas released in the cocurrent expansion step 3 to serve for pressure equalization and as a purge gas in a two-bed system, a total cycle time of 90 s is used, with the durations of the individual processing steps being as follows: step 1: 12 s, step 2: 28 s, step 3: 5 s, step 4: 32 s, step 5: 8 s and step 6: 5 s. The upper adsorption pressure is 150 kPa, the lower desorption pressure is 50 kPa, the pressure decrease at pressure equalization is to 110 kPa and the pressure increase at pressure equalization is to 85 kPa. Feed air is introduced at the upper adsorption pressure at a rate of 0.133 mol/s at 300°K, producing 0.039 mol/s of oxygen per cycle and recovering 0.021 mol/s as 95% purity oxygen product and using 0.010 mol/s as purge oxygen. The desiccant layer is at a temperature of about 300°K while the lower feed end of the nitrogen adsorption region is at a temperature of about 270°K as a result of the self-cooling feature of the invention and the upper product end of the nitrogen adsorbent region is at a temperature of about 298°K. The total cooling per unit of frontal area is about 7580 W/m², i.e. 79.8 kW for a bed of 3.66 m (12 ft) diameter. With the 5.1 cm (2 inch) thick insulation of the nitrogen adsorbent region according to the invention and with a lateral area of the vessel wall of 21 m², the heat loss through the wall of the nitrogen adsorbent region was kept to only 1.1 kW, with the corresponding temperature effect being only 0.4 ºK.

It will be understood that various changes and modifications may be made in the details of the invention as described herein without departing from the scope of the invention as defined in the appended claims. Thus, in addition to the alumina described above, the desiccant layer may contain silica gel, molecular sieve material such as certain Na-X materials having a high Si/Al ratio, e.g., 20/1, and the like.

The purpose of insulation on the side walls of the adsorbent vessel, i.e. in the area of the regenerator, dryer and adsorbent sections, is to prevent undesirable loss of self-cooling through the side walls. For adsorbent systems where the adsorbent beds are cylindrical vessels and the gas flows are axial, the side walls consist of the cylinder shell. If external insulation is used, it may be necessary to insulate not only the cylinder shell but also parts of the vessel manifold spaces to prevent heat conduction from these spaces to the shell wall. If the adsorbent bed is designed for radial flow rather than axial flow, the top and bottom are desirably insulated. It will be understood that the insulation used for such cylindrical containers, or for any other containers, should be of sufficient thickness and sufficiently low conductivity so that the total heat conduction into the adsorbent container through the walls is a very small fraction of the inherent cold generated during the cyclic PSA operations of the invention. Heat conduction into the container is thus minimized by the use of insulation, such heat conduction being less than about 5%, preferably less than about 2%, i.e., between about 1% or less and about 2% of the inherent cold generated within the container. It will be understood that any suitable commercially available insulation material may be used. Thus, readily available vacuum insulation, pipe insulation, and the like may be used, with insulation materials such as diatomaceous earth, silica, and the like being conveniently used.

For an axial flow adsorber of the type shown in FIG. 1, the regenerator is conveniently in the form of a flat sheet at the feed end of the adsorber. In a radial flow adsorber in which the gas flow is either from the center outward or from the edge toward the center, the regenerator is typically in the form of a cylindrical sheet which separates the feed end manifold assembly, either externally or internally, from the adsorbent region, the gas streams thus flowing radially through the regenerator.

In addition to the 0.8 cm (5/16 inch) conductive brass balls used to fill the lower distributor assembly in the above illustrative example, other suitable particles, including conductive particles such as alumina, may be used for such purposes. During pressurization, the gas in the distributor space is heated by means of reversible compression work. Heated gas is then forced through the regenerator and into the adsorbent bed. Eventually, the gas in the distributor space approaches the temperature of the feed gas entering the feed compressor. and exits its aftercooler. At this point, excess heat has entered the adsorber. During desorption, the gas in the plenum is cooled by reversible expansion work. Cooled gas exits through an outlet line and a vacuum pump, if used. Eventually, the gas in the plenum approaches the temperature of the gas leaving the warm end of the regenerator. This temperature is nearly the same as the temperature of the gas that left the feed compressor and its aftercooler, as long as the regeneration is operating effectively.

The net effect of the process is that reversible compression and expansion work in the end space acts as a heat pump. The heat from the pumping equipment is injected into the adsorbent, thereby canceling out at least some of the adsorbent's self-cooling. The heat pumped into the adsorbent travels through the entire bed in cycles, and it ultimately increases the temperature of the entire bed. Such undesirable pumping of heat into the distributor assembly at the feed end can be reduced by reducing the volume of the distributor space and/or partially filling the distributor space with rigid or bulky particles or structures, such as the brass balls mentioned above, which reduce the gas-filled void space. The remaining empty space of the manifold space is suitably about 40% of the volume of the unfilled manifold space, although it is understood that the remaining volume can be reduced to any volume which provides for an effective reduction in the pumping of heat at the manifold assembly or manifold space at the end of use. If the particles or structures for filling the manifold space have sufficient heat capacity and sufficient heat transfer surface, they can reduce the heat pump effect not only by reducing the gas-filled volume but also by dampening the temperature fluctuations of heat conduction with the gas therein. In this way they can absorb heat from the gas during compression and release this heat into the gas during expansion.

Although a heat pump effect also occurs in the distributor assembly at the product end at the opposite end of the adsorber, this does not significantly affect the adsorber temperature. While the distributor assembly is preferably filled with the particles or structures according to the preferred embodiments of the invention, there is no need to reduce the thermal cycle at the distributor space at the product end. This product end or upper distributor space may nevertheless be filled with ceramic balls or other particles or structures if a reduction in the loss of compression energy in the distributor space at the product end is desired. For those skilled in the art It will be understood that the invention may be practiced using a variety of process conditions depending upon the gas separation being carried out, the number of adsorbent beds and adsorbent used, the desired product properties, and the like. It is within the scope of the invention to use lower desorption pressures of from about 40.5 kPa to about 121.6 kPa (about 0.4 to about 1.2 atmospheres (atm)) in various embodiments of the invention. The ratio of upper adsorption pressure to lower desorption pressure is in the range of from about 1.25 to about 5.0 in accordance with the invention. It should be noted that within such operating ranges, the invention is preferably practiced within two separate operating ranges. Accordingly, in certain embodiments, the invention is advantageously practiced at lower desorption pressures of about 40.5 to about 70.9 kPa (about 0.4 to about 0.7 atm), e.g., 55.7 kPa (0.55 atm), wherein the ratio of upper adsorption pressure to lower desorption pressure varies from about 1.4/1 to about 5.0/1, preferably from about 1.7/1 to about 3.0/1. In other embodiments, the invention is advantageously practiced at lower desorption pressures of about 101.3 to about 121.6 kPa (about 1.0 to 1.2 atm), wherein the ratio of upper adsorption pressure to lower desorption pressure varies from about 1.25/1 to about 3.5/1, preferably from about 1.4/1 to about 2.5/1. In PSA cycles in which each bed is depressurized to an intermediate pressure before being depressurized to the lower desorption pressure, e.g. by means of the cocurrent depressurization step described above, the difference between the upper adsorption pressure and the intermediate pressure, or intermediate pressures if more than one intermediate pressure is used, is preferably between near 0% and about 40% of the total difference between the upper adsorption pressure and the lower desorption pressure used. The temperature of the air or other feed gas is suitably in the range of about 280ºK to about 310ºK, typically from about 290ºK to about 305ºK, with ambient temperature conditions being suitably. As mentioned above, the adsorbents used in the practice of the invention are equilibrium type Na-X zeolite molecular sieves which are only moderately strong adsorbents for the more readily adsorbable component of the gas mixture, e.g. nitrogen in air separation processes. Such adsorbents, e.g. Na-X with a ratio of silica to alumina as mentioned above, and the known materials of type 5A and 4A, thus show only moderate selectivity with respect to the nitrogen or the more selectively adsorbable component, only moderate adsorbent loading and only moderate heat of adsorption. In contrast, Li-X, Ca-X and other zeolites prepared by ion exchange of Na-X zeolites are strongly adsorbent with respect to nitrogen and the other more readily adsorbable components of the feed air or other feed gas, and they show high selectivity with respect to nitrogen. or the more selectively adsorbable component, together with a high loading and a large heat of adsorption.

In embodiments of the invention in which a purge step is used, as in other PSA cycles, it is understood that the adsorption front of the more selectively adsorbable component moves from the feed end of the bed to the product end of the bed during the high pressure adsorption step and the cocurrent expansion step, but without breakthrough from the product end of the bed. The amount of purge gas used during the lower desorption pressure is selected so that the desorption and removal of the more selectively adsorbable component from the feed end of the bed occurs without breakthrough of the less adsorbable component, i.e. oxygen in air separation applications, from the feed end of the bed. In applications with lower adsorption pressure to desorption pressure ratios, it is generally desirable to use sufficient purge gas to ensure desorption and removal of the more easily adsorbable component from the bed to the greatest extent possible without breakthrough of the less easily adsorbable component.

While the invention has been particularly described with respect to PSA air separation processes for the selective adsorption of nitrogen and for the recovery of the less adsorbable oxygen, or oxygen and argon, as the desired product, other gas separation processes may be advantageously carried out in the practice of the invention. It should be understood that air separation PSA cycles are known and may be used in conjunction with the invention wherein the more selectively adsorbable component, i.e. nitrogen, is the desired product and is recovered in the desorption portion of the PSA cycle with or without the use of a purge step. Other gas separation PSA processes which may be improved by the invention include the separation of nitrogen from helium or from hydrogen, nitrogen being the more readily adsorbable component of such gas mixtures.

Using the air separation and oxygen recovery system described in the above illustrative example, it has been found that performance comparable to that obtained with the external cooling approach can be achieved in the practice of the invention without the need for external cooling. It has also been determined that product yield is higher and bed size factor is desirably lower when the desorption pressure is kept low at fixed upper adsorption pressure conditions. However, increasingly low desorption pressures increase the power requirements of the vacuum pump required to achieve the vacuum desorption pressure levels. Thus, a moderate desorption pressure value and a moderately high Ratio of adsorption pressure to desorption pressure within the ranges specified above is generally desirable. The invention also provides highly desirable process flexibility, allowing balancing of operating characteristics with the requirements and constraints associated with a given application.

The invention represents a highly desirable advance in the field of PSA based on the effective utilization of the self-cooling generated during cyclic PSA operations. As a result, improved gas separations, such as in the highly desirable PSA operations for producing oxygen by air separation, can be achieved under optimal temperature conditions without the need and expense of external cooling sources. Through the practice of the invention, PSA technology will be enabled to more fully meet the needs and desires in the art for overall efficiency and overall economy, thereby meeting the ever-increasing demands of air separation for oxygen production and other desirable gas separation operations.

Claims (27)

1. Pressure swing adsorption system for separating a more easily adsorbable component from a feed gas mixture containing said component and a less easily adsorbable component, provided with: a) at least one adsorption vessel (1) containing a bed (7) of equilibrium adsorbent capable of selectively adsorbing the more easily adsorbable component of the feed mixture, the adsorbent bed having no means for supplying externally provided cold; b) a line arrangement (2) for transferring the feed gas mixture to the feed end of the adsorption vessel (1) and for withdrawing the more easily adsorbable component therefrom during desorption of the adsorbent bed (7); c) a line arrangement (9) for withdrawing the more easily adsorbable component from the opposite end of the container (1); d) distributor arrangements (3, 8) provided at the insert end and the opposite end of the adsorption vessel (1) adjacent to the line arrangement (2, 9), the adsorbent bed (7) being arranged between the distributor arrangements; e) a regenerator section (4) within the adsorption vessel (1), the regenerator section comprising conductive elements designed to store the cold in the returning stream during the desorption part of the adsorption/desorption processing sequence carried out in the bed (7); and f) an insulation arrangement on the walls (1A) of the adsorption vessel (1) designed to prevent the loss of any significant amount of self-cooling generated during the cyclic adsorption/desorption operation of the system due to the penetration of heat from outside the adsorption vessel through its walls to the adsorbent bed (7) or the regenerator section (4) within the vessel; g) wherein the adsorbent bed (7) is arranged downstream of the regenerator section (4); characterized in that h) the regenerator section (4) is arranged at the feed end of the adsorption vessel (1) directly downstream of the lower distributor arrangement (3); and i) the adsorbent is selected from the group consisting of sodium X zeolites having a silica/alumina ratio of 2.0 to 2.6, 5A zeolites and 4A zeolites, which exhibit a water uptake of less than about 25 g water per kg. 2. System according to claim 1, provided with a dryer section (5) arranged within the adsorption vessel (1) between the regenerator section (4) and the adsorbent bed (7) which dryer section comprises a drying agent capable of removing water vapor, carbon dioxide and heavy hydrocarbons from the feed gas mixture, wherein the region of the dryer section in the vicinity of the adsorbent bed serves as a second regenerator section (6). 3. System according to claim 1 or 2, wherein the silica/alumina ratio is less than 2.4. 4. System according to claim 1, wherein the regenerator section (4) comprises metal particles. 5. The system of claim 1, wherein the regenerator section (4) comprises particles or elements having a thermal conductivity in the range of about 430 to about 0.85 W/m K (about 250 to about 0.5 BTU/ºF/ft/ft²/hr). 6. The system of claim 5, wherein the thermal conductivity is between about 260 and about 26 W/m K (about 150 and about 15 BTU/ºF/ft/ft²/hr). 7. System according to one of the preceding claims, in which the water absorption is less than about 10 g water per kg. 8. A system according to any preceding claim, wherein the system comprises two or more adsorption vessels (1) containing the adsorbent bed (7), and the conduit arrangement (2) for passing the feed gas mixture to the system and for withdrawing the more readily adsorbable component from the system, and the conduit arrangement (9) for withdrawing the more readily adsorbable component from the system are adapted to carry out the adsorption/desorption pressure swing adsorption sequence in each bed on a cyclic basis. 9. System according to one of the preceding claims, in which the distribution arrangement (3) at the insert end of the adsorption vessel (1) is filled with particles which serve to substantially reduce the pore volume therein. 10. The system of claim 9, wherein the particles comprise conductive particles. 11. Pressure swing adsorption process for separating a more easily adsorbable component from a feed gas mixture containing said component and a less easily adsorbable component in a cyclic adsorption/desorption sequence, in which: (a) the feed gas mixture is passed at an upper adsorption pressure to the feed end of at least one adsorption vessel (1) containing a bed (7) of equilibrium adsorbent capable of selectively adsorbing the more readily adsorbable component of the feed gas mixture and which is selected from the group consisting of sodium X zeolites having a silica/alumina ratio of 2.0 to 2.6, 5A zeolites and 4A zeolites and exhibits a water uptake of less than about 25 g water per kg, wherein no externally supplied cold is supplied to the adsorbent bed, wherein the feed gas mixture is passed through a distributor arrangement at the feed end of the adsorption vessel (1) and through a regenerator section (4), wherein the adsorbent bed is arranged downstream of the regenerator section, wherein the regenerator section has conductive elements designed to store the cold in the return flow during the desorption portion of the adsorption/desorption processing sequence carried out in the adsorbent bed; the regenerator section being located at the feed end of the adsorption vessel immediately downstream of the lower manifold assembly (3); the adsorption vessel being provided with an insulation arrangement on its walls (1A) to prevent the loss of any significant amount of self-cooling generated during the cyclic adsorption/desorption operation of the system due to the penetration of heat from outside the adsorption vessel through its walls to the adsorbent bed or to the regenerator section within the vessel; (b) the less easily adsorbable component is withdrawn from the distributor arrangement (8) at the opposite end of the container (1) at the upper adsorption pressure; and (c) the more easily adsorbable component of the feed gas mixture is withdrawn from the feed end of the bed (7), the bed being expanded to the lower desorption pressure in the range from about 0.4 to about 1.2 bar (atmospheres) and the ratio of the upper adsorption pressure to the lower desorption pressure being in the range from about 1.24/1 to about 5.0/1. 12. The process of claim 11, wherein the feed gas mixture is passed through a dryer section (5) disposed within the adsorption vessel (1) between the regenerator section (4) and the adsorbent bed (7), which dryer section comprises a drying agent capable of selectively adsorbing water vapor, carbon dioxide and heavy hydrocarbons from the feed gas mixture, the region of the dryer section proximate the adsorbent bed serving as a second regenerator section (6). 13. The process of claim 11 or 12, wherein the lower desorption pressure is between about 0.4 and about 0.7 bar (atmospheres) and the ratio of the upper adsorption pressure to the lower desorption pressure is in the range of about 1.4/1 to about 5.0/1. 14. The method of claim 13, wherein said pressure ratio is in the range of about 1.7/1 to about 3.0/1. 15. The process of claim 11 or 12, wherein the lower desorption pressure is between about 1 and about 1.2 bar (atmospheres) and the ratio of the upper adsorption pressure to the lower desorption pressure is in the range of about 1.25/1 to about 3.5/1. 16. The method of claim 15, wherein said pressure ratio is in the range of about 1.4/1 to about 2.5/1. 17. The method of any one of claims 11 to 16, wherein the thermal conductivity of the particles of the regenerator section is between about 430 and about 0.85 W/m K (about 250 and about 0.5 BTU/ºF/ft/ft²/hr). 18. The method of claim 17, wherein the thermal conductivity is between about 260 and about 26 W/m K (about 150 and about 15 BTU/ºF/ft/ft²/hr). 19. A process according to any one of claims 11 to 18, wherein the water absorption is less than about 10 g water per kg. 20. A process according to any one of claims 11 to 19, wherein the silica/alumina ratio is less than 2.4. 21. A method according to any one of claims 11 to 20, wherein the distributor arrangement at the insert end of the adsorption vessel contains particles which serve to appreciably reduce the pore volume therein. 22. A process according to any one of claims 11 to 21, wherein the feed gas mixture comprises air. 23. A process according to claim 22, wherein the less readily adsorbable component recovered from the opposite end of the bed (7) comprises oxygen and argon product gas. 24. A process according to claim 22, wherein the more readily adsorbable component recovered from the feed end of the bed (7) comprises nitrogen product gas. 25. A process according to any one of claims 11 to 21, wherein the feed gas mixture comprises nitrogen as the more easily adsorbable component and helium as the less easily adsorbable component. 26. A process according to any one of claims 11 to 21, wherein the feed gas mixture comprises nitrogen as the more easily adsorbable component and hydrogen as the less easily adsorbable component. 27. A process according to any one of claims 11 to 21, wherein the feed gas mixture is fed to two or more adsorption vessels (1) on a cyclic basis.